1. Field of the Invention
The present invention relates to a method for manufacturing a capacitor of a semiconductor device, and in particular to an improved method for manufacturing a capacitor employing doped silicon film as an electrode and oxide film-nitride film-oxide film (hereinafter referred to as “ONO film”) as a dielectric film, wherein the thickness of the nitride film in the ONO film can be effectively reduced and a characteristic of the breakdown voltage due to leakage current can be improved.
2. Description of the Background Art
Generally, a capacitor in a DRAM memory cell functions as a storage for electric charge to store information. Therefore, the capacitor requires sufficient capacitance and high reliability in long term repeated use, and the dielectric film must exhibit good insulation properties so there is low leakage current.
As the integration density of devices increases, the area allocated to a unit cell is reduced, resulting in difficulties in obtaining sufficient capacitance in a capacitor. Therefore, in order to increase the capacitance of a capacitor, the height of the capacitor must be increased and the process margin between adjacent cells must be reduced.
The capacitors in a DRAM cell must have a capacitance of at least 25 fF. Since the capacitance of a capacitor is proportional to the area of the surface of the storage electrode and inversely proportional to the thickness of a dielectric film, methods for increasing the area of the capacitor or decreasing the thickness of the dielectric film have been proposed.
An oxide film having a dielectric constant of 3.8 was used as a dielectric film early in the development of capacitors in semiconductor devices. Subsequently, a nitride film having a dielectric constant of 7 was used. Presently, novel dielectric materials such as Ta2O5, Al2O3 and HfO2 are used in devices having capacity of more than 256 Mbits.
As for the capacitor structure, a stack structure was employed in early devices. However, a cylinder type or concave type capacitor, having a MPS (meta-stable polysilicon) for increasing the area of the storage electrode by 1.7˜2 times, is currently used because the use of a stack structure became difficult due to high integration density.
Although not shown in the figures, a conventional method for manufacturing a capacitor of semiconductor device using doped silicon as electrode is as follows.
An interlayer insulating film having a contact plug for a storage electrode therein is formed on a semiconductor substrate. Next, a storage electrode is formed on the interlayer insulating film. A natural oxide film on the storage electrode is then cleaned using a solution containing HF. A first oxide film is then formed on the storage electrode in an O2, H2O or O2 atmosphere. The first insulating film may be formed by performing a wet oxidation process using a mixture solution of NH4OH and H2O2.
Thereafter, a nitride film is formed on the first insulating film and the nitride film is then oxidized in a H2O atmosphere to form a second oxide film therein. Next, a plate electrode is formed on the second oxide film using doped polysilicon.
In accordance with the conventional method for manufacturing a capacitor of a semiconductor device, the natural oxide film on the storage electrode is cleaned using a solution containing HF. The solution containing HF not only removes the natural oxide film, but also removes dopants in the storage electrode consisting of doped polysilicon, thereby causing charge depletion. Moreover, when the thickness of the nitride film is reduced to increase permittivity of the dielectric layer, the breakdown voltage of the dielectric layer is drastically reduced.
The problem of charge depletion appears more severely in cylinder type capacitors because the surface of the storage electrode is additionally exposed to an etching solution during the process for removing a sacrificial oxide film.